1. Field of the Invention
This invention relates to methods for making reinforced, polymeric articles in reaction injection molding systems, molds for use therein and articles produced thereby.
2. Background Art
Reaction injection molding (RIM) is a process by which highly chemically reactive liquids are injected into a mold where they quickly polymerize to form a coherent, molded article. Some of the most common RIM processes involve a rapid reaction between highly catalyzed polyether or polyester polyol and isocyanate constituents. The constituents are stored in separate tanks prior to molding and are first mixed in a mixhead upstream of a mold. Once mixed, they react rapidly to gel and then harden to form polyurethane polymers.
As described in U.S. Pat. No. 5,753,155, rigid foam applications have struggled for many years with problems caused by liquid turbulence and splashings of the chemical stream into the part being foamed. The net result of this turbulence is the entrainment of air in the chemical stream, splashing of chemicals onto the internal part surfaces, uneven distribution of chemicals, and mixing of already reacting chemicals with new materials being introduced into the part. Depending on the size and complexity of the part, this results in surface voids, air entrapment, weak sub-surface areas and flow restrictions. The consequences can be poor surface quality, reduced physical properties, and potential of blister formation or even catastrophic failure.
In the rigid and flexible molded area, this turbulence has, in some cases, been reduced by specially designed mixheads. Unfortunately, these mixheads are very costly, generally lose their effectiveness with high output foam machines, and often cannot be used because of their large size and complexity. In the RIM area, a sophisticated gating technology has been developed to assist in mixing and directing liquid chemicals so that these flow into molds in laminar fashion (see, e.g., U.S. Pat. No. 3,991,147). These so-called gates are typically precision-machined into the mold or into a steel block permanently attached to the mold.
Although reaction injection molded urethanes have many desirable physical characteristics, they also have generally high coefficients of thermal expansion (CTE), poor dimensional stability over wide temperature ranges and considerable flexibility at room temperature. Moreover, a large, filler-free RIM panel when attached to a rigid support structure may permanently buckle and wave at elevated temperatures. Thus, as molded, unreinforced RIM urethanes are not generally directly suitable for use as large automotive panels or in other semistructural or structural panel applications. Furthermore, the larger the surface area and thinner the aspect of a panel, the more serious these problems become.
The use of reinforcing fillers in RIM urethanes has been extensively examined. For example, reinforced reaction injection molding (RRIM) technology has been utilized for 25 years for automotive fascia and body side moldings. More recently, RRIM has been used for pick-up truck fenders. Today's applications demand high performance in processing and part performance. Parts molded in polyurethane RRIM systems such as the Bayflex® 190 elastomeric polyurethane/polyurea RRIM system of the Bayer Corporation of Pittsburgh, Pa., have difficulty in maintaining laminar-like flow during mold filling, especially for large exterior automotive body panels such as car or truck fenders where as much as 18 pounds of material flow into the article-forming cavity within a mold in less than a second. As a result, flow lines or surface waviness can be seen in the gate area of the part using conventional RIM gating systems. These lines result in an unacceptable part, particularly noticeable when the part is coated with glossy paint.
As described in detail in U.S. Pat. No. 4,648,832, FIG. 1 illustrates a prior art after-mixer and gating apparatus, generally indicated at 10 and generally including an after-mixer block or body 12 and a gating block or body 14 positioned in one or the other of the mold halves for further mixing the constituents of RIM material and admitting the RIM material to a mold cavity 16.
The after-mixer body 12 includes at least one common inlet passageway 18 for receiving the RIM material from a high-pressure mixing apparatus (not shown) at a predetermined flow rate and pressure. The RIM material flows through the inlet passageway 18 into at least one pair of curvilinear mixing passageways 22 and 24 where it is separated into a corresponding number of flow streams. Such separation increases the turbulence of the RIM material flow and accordingly enhances the intermixing of its constituents. The mixing passageways 22 and 24 include an inverted U-shaped divider wall 26 therein, which form a generally serpentine path in the mixing passageways 22 and 24. Such serpentine path includes at least one bight portion 28 in the mixing passageway 22 and a similar bight portion 30 in the mixing passageway 24. Such bight portions 28 and 30 cause the RIM material to substantially reverse its direction of flow, as indicated by the flow arrows 32 and 34, respectively, in order to increase the turbulence of each of the RIM material flow streams and enhance the intermixing of the constituents therein.
Once the RIM material flows through the mixing passageways 22 and 24, it enters a corresponding number of respective inlet openings 36 and 38 of at least one common mixing cavity 40. The flow streams from the mixing passageways 22 and 24 are reunited in the mixing cavity 40 which again reverses the flow directions of the flow streams, increases their turbulence, and enhances the intermixing of the material constituents. The reunited and intermixed material from the mixing cavity 40 is received by an inlet passageway 42 in the gating body 14, wherein it is conveyed through a diverging fan-gate passageway 44 to the mold cavity 16.
The following U.S. patents disclose various RIM systems: U.S. Pat. Nos. 4,564,491; 4,729,863; 5,346,387; 5,681,867; 5,770,141; 6,197,242; and 6,221,290.
The following U.S. patents disclose various gating systems for molds: U.S. Pat. Nos. 4,618,322; 5,098,281; 5,409,654; 5,422,059; 5,423,673; 5,728,343; and 6,060,004.
However, despite all of the above-noted prior art, the above-noted problem still exists.